This invention relates to a vapor drier to dry an object to be processed such as a wafer for semiconductor devices by using vapor of volatile processing liquid.
In a production process of semiconductor devices, for instance, after a semiconductor wafer is processed by particular chemicals, the chemicals are removed by washing with water and the water deposited on the wafer is removed by drying. A vapor drier is used as a device for drying semiconductor wafers.
In this type of vapor drier, isopropyl alcohol (IPA) as volatile processing liquid is put into the bottom section of a drying room with an opening at the top, a heater is set under the processing room, the IPA is heated and evaporated by this heater to fill the processing room with vapor atmosphere of this IPA, and a semiconductor wafer to be processed is exposed to the IPA vapor atmosphere to be dried. The semiconductor wafer is generally guided into the processing room together with a carrier tool. Also in the upper section of the processing room is arranged a cooling means for liquefying IPA vapor that comprises, for instance, a cooling coil arranged inside the top opening and other components.
The drying process can be explained as follows. As the wafer guided into the processing room has been washed with pure water at the room temperature in the previous process, much water is deposited on this semiconductor wafer when said wafer is guided into the processing room, and furthermore temperature of the semiconductor wafer is almost the same as the room temperature. For these reasons, when the wafer is guided into the processing room, the IPA vapor is condensed and liquefied on the surface of the wafer due to the difference of the temperature between the wafer and hot IPA vapor atmosphere. And the water deposited on the surface of the wafer is dissolved into the liquefied IPA, and the dissolved materials gradually go off from the surface of the wafer, so the wafer is dried by and by.
As a prior document disclosing this type of vapor drier, for instance, Japanese Laid-Open Patent Publication No. 58-207638 can be listed.
In the structure of the conventional vapor drier as described above, water deposited on wafers or the carrier tool, moisture in the air which is condensed on the surface of the cooling coil in the upper section of the processing room, and impurities are accumulated in the processing room when the wafers and the carrier tool are guided into the processing room. For this reason, as the wafer drying process is repeated many times, composition of IPA in the processing room largely changes. In other words, in the conventional type of devices, a content of water and a content of impurities in IPA in the processing room increase as time passes.
Here, increase of water content in IPA in the processing room becomes naturally a great cause for failures in the wafer drying process. Note that logically water content continues to increase to an azeotropic point between IPA liquid and water. Furthermore, if a content of impurities in the IPA liquid is large, when the IPA liquid is heated and boiled by the heater and splashes are generated, the impurities (foreign materials) deposit on the surface of the wafer, which also causes a failure in the drying process.
In order to reduce the probability of generation of failures in the conventional wafer drying process, an operation of the vapor drier is stopped periodically, and IPA liquid residing in the processing room is discharged and exchanged with fresh liquid, while operation of the vapor drier is down.
However, although failures in the wafer drying process can be reduced by frequently exchanging IPA liquid, as practical operating time of the vapor drier is reduced every time the hot IPA liquid is exchanged, throughput in the wafer process becomes lower.
Also, if the work to exchange the IPA liquid must be made frequently, workers are exposed to the IPA liquid many times, which is problematic for health control and economically disadvantageous because use rate of the IPA liquid goes higher.
For these reasons, various examinations have been carried out to make the exchange interval of the IPA liquid as long as possible by taking into account allowance in the wafer process, or to reduce a content of water accumulated in the processing room by taking into account the structure of the processing room. However, these methods are temporary, and can not be substantial solutions.
A purpose of this invention is to provide a vapor drier which can continue a process to dry efficiently objects to be dried in an extremely clean condition for a long time.